1. Field of the Invention
The present invention relates to joining of carbon steel pipes, and more particularly to joining of oil well pipes, such as casing tubes, production tubes or coiled tubes, for use in an oil well or a gas well, joining of pipes for a plant or line pipes for use in the chemical industry, petrochemistry or the like, and a method of enlarging the inner diameter of the joint of carbon steel pipes.
2. Description of the Related Art
conventionally, the oil well pipe, such as the casing tube or the production tube, for use in an oil well or a gas well, has been constituted by joining a multiplicity of carbon steel pipes each having a predetermined length (about 10 m to about 15 m).
For example, when an oil well or a gas well is mined, steel pipes, called casing tubes, are embedded in the shaft to protect the shaft dug in the ground and prevent leakage of crude oil or gas. Since the oil well or the gas well is usually present thousands of meters below the ground or the bottom of the sea, the casing tube must have a length of thousands of meters. Crude oil or gas pumped up from the oil well or the gas well is transported to an accumulating tank or a refinery through a separating unit by using a flow line or a pipe line extended for a distance of several kilometers.
The carbon steel pipes for use in the foregoing case are joined by any one of known methods below: a screw joining method (a mechanical coupling method), a welding method (an orbital welding method), a friction welding method or a diffusion bonding method.
The screw joining method is a method arranged to engage screws formed at the ends of the carbon steel pipes to join the carbon steel pipes to each other. The screw joining method has an advantage that time required to join each pipe takes a short time of about 5 minutes to 10 minutes and satisfactory working efficiency can be realized. The foregoing method, however, suffers from a problem in that oil or gas easily leaks through the joined portion. It leads to a fact that the screws provided for the carbon steel pipes must have excellent accuracy. Moreover, excellent skill is required to perform the screw joining operation. To protect the screw portion formed with excellent accuracy from damage, transportation and handling of the carbon steel pipes having the screw portions require close attention to be paid. Although airtightness of the joined portion can be maintained in a case where tensile stress is exerted, the screw joined portion is undesirably enlarged in the radial direction when compressive stress is exerted. Thus, there arises a problem in that leakage of oil or gas cannot be prevented.
In the welding method, a beveling is provided for the end surface of each of the carbon steel pipes and the beveling is filled with metal which must be welded so that the carbon steel pipes are connected to each other. The welding method is free from any leakage of oil or gas from the welded portion if incomplete fusion is performed or a blow hole is not formed. Moreover, an advantage can be realized in that satisfactory welded portion permits equivalent characteristics to those of the base metal to be obtained against compressive stress as well as tensile stress. The welding method, however, suffers from unsatisfactory efficiency. Since multilayer welding must be performed in a case of thick pipes each having a large diameter, an excessively long time of one to two hours is required to complete one joint. Moreover, there arises a problem in that influences of the environment including the weather and wind are exerted on the welding operation which is performed at the site. What is worse, excellent welding skills are required.
In the friction welding method, the abutted carbon steel pipes are relatively rotated or slid while pressure is being applied. Then, thus-generated fictional heat is used to soften and weld the ends of the carbon steel pipes to each other. The foregoing method is able to realize advantages in that any skill is not required, the joining operation can be completed in a short time and excessive influence of the operation environment can substantially be eliminated. However, the foregoing method cannot prevent flash of the inner and outer surfaces of the joined portion of the carbon steel pipes. Therefore, there arises a problem in that removal of the flash, in particular, removal of the flash formed on the inner surface requires a long time. As a method to solve the foregoing problem, a radial friction welding method has been developed. In the method, a ring having a wedge-like cross section is inserted between the end surfaces of a pair of carbon steel pipes while securing the pair of the carbon steel pipes, the ring is inwards pushed toward the center of the carbon steel pipe to perform pressure-welding. The pressure-welded joint attains only poor characteristic. Therefore, there is no report about the employment of the method to join the oil-well pipes.
The diffusion joining method is able to prevent any leakage of oil or gas through the joined portion if the joining operation is performed under appropriate conditions. Although resistance against compressive stress arises similarly to the foregoing welding method, time required to complete one joint can be shortened to about ⅓ to xc2xd of the welding method. Thus, advantage can be realized in that joints exhibiting high quality can efficiently be formed. Therefore, the diffusion joining method is a significantly advantageous method as a method of joining oil-well pipes and line pipes.
On the other hand, in order to reduce the oil or gas producing cost, a method has been developed with which a shaft having a diameter smaller than that of the conventional method is formed. Moreover, casing tubes each having a length of hundreds of meters and inserted into the shaft is expanded on the ground (World Oil: P.31, April, 1999).
In the foregoing method, a tool which has been inserted into a casing tube having a long length of hundreds of meters and which has an outer diameter larger than the outer diameter of the tube is moved in the tube by using hydraulic pressure or the like. Thus, the inner diameter of the tube is continuously enlarged. Therefore, the diameter of the shaft is reduced to reduce drilling cost. The distance from the shaft to the casing tube is shortened to reduce the cementing cost. Moreover, the number of the required casing tubes is decreased. Thus, reduction in the production cost of oil or gas is expected.
As described above, the casing tube having the long length of hundreds of meters cannot be constituted by one carbon steel pipe. Therefore, a multiplicity of carbon steel pipes each having a predetermined length are joined to one another to form a joint of carbon steel pipes and expanded. The joints of carbon steel pipes are connected to one another by any one of the foregoing methods, such as the above described screw joining method (the mechanical coupling method), the welding method (the orbital welding method), the friction welding method and the diffusion bonding method. Also the joined portion of the joint of carbon steel pipes is expanded similarly to the base metal of the carbon steel pipe. Therefore, the joined portion must have diameter-expansion characteristic similar to that of the base metal of the carbon steel pipe.
When the joints of carbon steel pipes connected by the screw joining method are expanded, deformation of the joined portion results to loose the clamped screws. Thus, there arises a problem in that oil or gas easily leaks. Therefore, precise screws each having a special shape are employed. However, the foregoing method cannot permit satisfactory characteristics to be obtained. Moreover, machining of the foregoing special screws requires a large cost. To protect the screw portions from damage, there arises a problem in that transportation or handling of the screws require close attention to be paid. Therefore, its applicable range is limited.
The mechanical characteristics of the welded portion of each of joints of the carbon steel pipes joined by the welding method are similar to those of the base metal if the welded portion is free from any defects, such as incomplete fusion or pores. To maintain the mechanical characteristics of the welded portion, a projection called xe2x80x9cweld reinforcementxe2x80x9d is formed on the outer surface of the welded portion of the carbon steel pipes. Moreover, projections called xe2x80x9cpenetration beadsxe2x80x9d are formed on the inner surface of the welded portion of the carbon steel pipe. Therefore, expansion of the joint of carbon steel pipes joined by the welding method using an expanding tool must have a process for removing the penetration beads on the inner surface of the welded portion of the carbon steel pipe which inhibits passing of the expanding tool. The removal of the penetration beads formed on the inner surface of the welded portions of the carbon steel pipes at the site in the oil well or the gas well requires a great cost and encounters difficulty. From a viewpoint of preventing defects, such as cracks of the welded portion in the expanding process, the ends of the weld beads on the outer surface of the welded portion of the carbon steel pipes must be smoothed. As an alternative to this, the weld reinforcement on the outer surface of the welded portion of the carbon steel pipes must be removed. There arises a problem in that also the foregoing operation requires a great cost.
The joint of carbon steel pipes which has been friction-welded by the friction welding method encounters formation of flash on the outer surface of the welded portion. Therefore, expansion of the joint of carbon steel pipes welded by the friction welding method by using an expanding tool must perform a process for removing at least flash on the inner surface of the welded portion of the carbon steel pipes. The removal of the flash formed on the inner surface of the welded portions of the carbon steel pipes and obstructing passing of the expanding tool at the site in the oil well or the gas well requires a great cost and encounters difficulty. From a viewpoint of preventing defects, such as cracks of the welded portion in the expanding process, the flash on the outer surface of the welded portion of the carbon steel pipes which serve as stress concentration source must be smoothed. Alternatively, the flash on the outer surface of the welded portion of the carbon steel pipes must be removed. There arises a problem in that the foregoing machining processes require a great cost.
The joint of carbon steel pipes joined by using the diffusion welding method does not suffer from formation of weld reinforcement on the outer surface of the welded portion of the carbon steel pipes and penetration beads formed on the inner surface of the same as distinct from the welding method. Moreover, formation of flash on the inner and outer surfaces of the welded portion of the carbon steel pipes can be prevented as distinct from the friction welding method. Therefore, when the joint of carbon steel pipes joined by the diffusion welding method is expanded by using the expanding tool, the necessity of removing the projections formed on the inner surface of the joint of carbon steel pipes which obstructs passing of the expanding tool can be eliminated as distinct from the welding method or the friction welding method. Moreover, the necessity of smoothing the projections formed on the outer surface of the joint of carbon steel pipes which serve as the stress concentration source or removing projections formed on the outer surface of the joint of carbon steel pipes in order to prevent occurrence of defects, such as cracks, of the welded portion in the expanding process can be eliminated. Therefore, an advantage can be obtained in that the expanding operation can significantly efficiently be performed.
When joint of carbon steel pipes is manufactured by using the diffusion joining method, the following factors are previously examined: the tensile strength of the joint of carbon steel pipes in a state where joint has been established and an influence of the joining temperature which is exerted on the characteristics, such as fatigue strength and the like, duration time at the joining temperature and the joining conditions, such the pressure, which are exerted on the joined surfaces. Thus, a range of the joining conditions which satisfies required characteristics is obtained to manufacture the joint of carbon steel pipes.
A situation will now be considered in which a tool having an outer diameter larger than the inner diameter of the joint of carbon steel pipes is inserted into the joint to expand the joint of carbon steel pipes. The joint of carbon steel pipes joined under condition satisfying the joining conditions obtained from a result of examination of the influence of the joining conditions which is exerted on the characteristic of the joint of carbon steel pipes in the joined state cannot eliminate fear of occurrence of defects, such as cracks of the joined portion, during the expanding operation. Moreover, satisfactory characteristics of the expanded joint of carbon steel pipes cannot be obtained.
If the joined portion of the joint of carbon steel pipes inserted into hundreds-meter-deep shaft and having a length of hundreds of meters is cracked, pressurizing fluid leaks through the cracks. It leads to a fact that the pressure for forwards moving the expanding tool is lowered. As a result, the expanding tool cannot furthermore be moved at the cracked portion. Hence it follows that the expanding operation cannot be continued. In the foregoing case, the foregoing problem cannot easily be restored. If the worst happens, the oil well or the gas well must be abandoned. Thus, a great loss is produced.
Further, if the joined portion of the joint of carbon steel pipes inserted into hundreds-meter-deep shaft and having a length of hundreds of meters is cracked, the joint of carbon steel pipes forwards than the position at which the cracks have occurred are sometimes separated. In the foregoing case, the expanding operation cannot be continued. What is worse, the shock sometimes causes the shaft to fall or the pressuring fluid is jetted from the gap between the shaft and the expanded joint of carbon steel pipes. Thus, a very dangerous state is realized. In the foregoing case, the foregoing accident cannot easily be restored. If the worst happens, the oil well or the gas well must be abandoned. Thus, a great loss is produced.
An object of the present invention is to provide a method of manufacturing a joint of carbon steel pipes suitable for a joining method in that carbon steel pipes are joined to one another in an appropriate joining condition range so as to expand the joint of carbon steel pipes by inserting a tool having an outer diameter larger than the inner diameter of the joint of carbon steel pipes into the joint to expand the joint of carbon steel pipes. Another object of the present invention is to provide a method of expanding a joint of carbon steel pipes which is free from an apprehension that defects, such as cracks, occurs in the joined portion during the expanding operation.
To solve the above-mentioned problems, a method of manufacturing a joint of carbon steel pipes suitable for expansion according to the present invention, as shown in FIG. 1, comprises: abutting carbon steel pipes (30) against each other; inserting an insert (31) having a melting point (Ti) lower than a melting point (Tb) of a base metal of the carbon steel pipes between joint surfaces of the carbon steel pipes; heating at least a portion adjacent to the joint portion to a temperature (T) expressed by Ti less than T  less than Tb; and pressurizing and holding the two carbon steel pipes (30) to diffusion-weld the two carbon steel pipes to each other, wherein the insert (31) comprises a Ni-base alloy or a Fe-base alloy having a melting point lower than 1200xc2x0 C. and a thickness of 80 xcexcm, and a joining operation is performed in a non-oxidizing atmosphere under conditions that surface roughness Rmax of the joining surfaces of the carbon steel pipes is 20 xcexcm or smaller, joining temperature is in the range of 1250xc2x0 C. to 1330xc2x0 C., duration at the joining temperature is 30 seconds or longer and applied pressure is in the range of 2 MPa to 4 MPa.
As shown in FIG. 2, according to the present invention, there is provided a method of expanding a joint of carbon steel pipes by using a tool (33) inserted into a joint (32) constituted by joining a plurality of carbon steel pipes, the method of expanding a joint (32) of carbon steel pipes comprising the steps of: using a joint (32) of carbon steel pipes incorporating joint portions each having a maximum difference in level which is not larger than 25% of the thickness of the carbon steel pipe; and making an expansion ratio of the inner diameter of the joint of carbon steel pipes (inner diameter (d1) of carbon steel pipe after expansionxe2x88x92inner diameter (d0) of carbon steel pipe before expansion)/inner diameter (d0) of carbon steel pipe before expansionxc3x97100%) to be 25% or lower.